GRIB stands for Gridded Binary File.
The first GRIB format is used worldwide by many meteorological centres for Weather Prediction. However, a newer generation has been introduced, known as GRIB second edition. Data is slowly changing over to this format.
The digital weather data provided in a GRIB file comes in raw form and is machine generated from one or more computer generated models. The weather data is generally provided in raw format by organisations such as the US National Weather Service (part of NOAA) and Meteo France and can be read by GRIB readers both online, by way of an app or other software.
The most data delivery systems are based on the US GFS System (General Forecast System) with other styles of delivery including PWG, PWE and ECMWF also used.
GRIB files include information including wave height, wind direction and speed and rainfall. The files, being machine generated, can be quite accurate for standard grid definition (usually ¼ of one degree of latitude or 15 nautical miles). This scale varies dependent on the GRIB reader used, although the original scale of the GFS source data remains the same.
GRIB apps such as Wind Guru work by interpolating data from the GFS and other models and interpolating for specific areas. Alternatively, you can import GRIB files into other software on board or at home for passage planning purposes.
GRIB files delivered by email can be quite an economic way of receiving weather data at sea. Files sizes might be circa 8 - 15 KB for an area covering the Mediterranean Sea. Great if you are using marine broadband.
It is worth noting that they are rarely accurate past 36 hours without some form of human interpolation in the interim. Grid size is also important, with very small scale representations (for example 1 km squares) are of limited use to sailors.
Weather routing services use GRIB files and other data / knowledge to interpolate and predict weather in specific areas.